Device and method for processing light-polymerizable material for building up an object in layers

ABSTRACT

A method and a device for processing light-polymerizable material for the assembly of a mold, utilizing a lithography-based generative manufacturing technique wherein a layer of a light-polymerizable material, the material being located in at least one trough ( 4 ) having a particularly light-transmissive, horizontal bottom ( 6 ), is polymerized by illumination on at least one horizontal platform ( 12 ), the platform having a prespecified geometry and projecting into a trough ( 4 ), in an illumination field, wherein the platform ( 12 ) is displaced vertically to form a subsequent layer, light-polymerizable material is then added to the most recently formed layer, and repetition of the foregoing steps leads to the layered construction of the mold in the desired form, which arises from the succession of layer geometries. The invention is characterized in that the trough ( 4 ) can be shifted horizontally to a supply position, and the supply device ( 8 ) brings light-polymerizable material at least to an illumination field of the trough bottom ( 6 ), before the at least one trough ( 4 ) is shifted to an illumination position in which the illumination field is located below the platform ( 12 ) and above the illumination unit ( 10 ), and illumination is carried out.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of International Application No. PCT/EP2008/008864 (Publication No. WO 2010/045950 A1), filed Oct. 20, 2008, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The invention is directed at the building up of objects and, in particular, to the building up of objects that are intended to be used for dental restorations.

BACKGROUND OF THE INVENTION

CAD-CAM technologies have been established in the dental sector for some time and have taken the place of the traditional manual crafting of tooth replacements. However, the methods customary today for producing ceramic dental restoration elements by removing material have several disadvantages, which cannot be improved with reasonable expenditure from economic aspects by the current state of the art. In this connection, building-up methods of production that are known under the term “rapid prototyping” can be considered, in particular stereolithographic methods in which a newly applied layer of material is respectively polymerized in the desired form by position selective exposure, whereby the desired body is gradually produced by shaping in layers in its three-dimensional form, which results from the succession of the layers applied.

With respect to ceramic-filled polymers, WO 98/06560, which is hereby incorporated by reference, should be mentioned in particular. In this case, a ceramic slip is exposed by way of a dynamic mask (light modulator), whereby a three-dimensional body is intended to be gradually built up. In the case of the method described, the ceramic slip is exposed from above on a build platform. In the case of such exposure from above, after each exposure a new thin layer of material must be applied with the aid of a doctor blade (typically with a layer thickness which lies between 10 and 100 μm). When using materials of relatively high viscosity, as ceramic-filled resins are, it is only with difficulty, however, that such thin layers can be applied in a reproducible manner.

In the prior art, there are also known techniques, at least for photomonomers without ceramic filling, in which the exposure takes place from below through the bottom of a vat, which is formed by a transparent film, sheet or sheet with an elastomeric surface (for example of silicone or fluoroelastomer). Above the transparent film or sheet there is a build platform, which is held at a settable height above the film or sheet by a lifting mechanism. In the first exposure step, the photopolymer between the film and the build platform is polymerized in the desired form by exposure. When the build platform is raised, the polymerized first layer becomes detached from the film or sheet and liquid monomer flows into the space created. The object polymerized in layers is created by successive raising of the build platform and selective exposure of the monomer material that has flowed in. A device suitable for applying this method is described for example in DE 199 57 370 A1, which is hereby incorporated by reference. A similar procedure is described in DE 102 56 672 A1, which is hereby incorporated by reference, which however likewise relates to unfilled polymers.

In the processing of ceramic-filled photopolymers, the following problems arise in comparison with the processing of unfilled photopolymers:

-   -   The green strength of the polymerized objects is significantly         lower (less than 10 MPa) than the strength of an unfilled         polymer (typically about 20 to 60 MPa). As a result, the         ceramic-filled photopolymer object can withstand significantly         less mechanical loading (for example when the last-formed layer         is detached from the sheet or film through which exposure was         performed from below).     -   The high proportion of ceramic particles causes pronounced light         diffusion, and the depth of penetration of the light that is         used is significantly reduced. Associated with this is         non-uniform polymerization in the z direction (direction of         radiation) in the case of layer thicknesses of more than 20 μm.         The small depth of penetration also makes it difficult to         achieve reliable bonding of the first layer directly on the         build platform. In the case of ceramic-filled monomer material,         however, it cannot be ensured that the initial starting layer is         sufficiently thin (for example less than 75 μm). Consequently, a         reproducible bonding force on the build platform could not be         ensured even with very long exposure of the first layer.     -   In comparison with unfilled photopolymers, ceramic-filled         polymerizable materials are significantly more viscous. This         imposes increased requirements on the exposure mechanism that is         used. In particular, the time that is required for         ceramic-filled photopolymer to flow in after raising of the         build platform may be considerably longer. The raising and         lowering of the build platform in a highly viscous photopolymer         material also imposes increased requirements to avoid         detrimental effects on the component.     -   On account of the high basic viscosity, ceramic-filled         photopolymers are more sensitive to gelling by diffused light or         ambient light. Even small light intensities are sufficient to         raise the viscosity of the material above the permissible limit         by the polymerization taking place.

The problem addressed by the present invention is that of improving a building method and a device for processing light-polymerizable materials for building up objects, using lithographic rapid prototyping, in such a way that they allow even light-polymerizable materials of relatively high viscosity, in particular ceramic-filled photopolymers, to be processed better.

SUMMARY OF THE INVENTION

The device and methods according to the independent patent claims serve for solving this problem. Advantageous embodiments of the invention are specified in the subclaims.

The invention relates to a device for processing light-polymerizable material for building up an object in layers, using lithography-based generative fabrication, for example rapid prototyping, with a vat with an at least partially transparently or translucently formed horizontal bottom, into which light-polymerizable material can be filled, a horizontal build platform which is held at a settable height above the vat bottom, an exposure unit, which can be controlled for position selective exposure of a surface on the build platform with an intensity pattern with predetermined geometry, a control unit, which is arranged for polymerizing in successive exposure steps layers lying one above the other on the build platform, respectively with predetermined geometry, by controlling the exposure unit and for adjusting, after each exposure step for a layer, the relative position of the build platform to the vat bottom, in order in this way to build up the object successively in the desired form, which results from the sequence of the layer geometries.

The invention also relates to a method for processing light-polymerizable material for building up an object, using a lithography-based generative fabrication technique, for example rapid prototyping, in which a layer of a light-polymerizable material which is located in at least one vat with an in particular transparently or translucently formed, horizontal bottom is polymerized on at least one horizontal build platform, protruding into at least one vat, with predetermined geometry by exposure in an exposure area, the build platform is displaced vertically for the forming of a subsequent layer, light-polymerizable material is newly fed onto the layer last formed, and, by repeating the previous steps, the object is built up in layers in the desired form, which results from the sequence of the layer geometries.

BRIEF DESCRIPTION OF THE FIGURES

Further advantages, details and features emerge from the following description of embodiments of the invention on the basis of the drawings, in which:

FIG. 1 shows a lateral plan view, partly in section, of a device according to the invention,

FIG. 2 shows a plan view of the device from FIG. 1, from above,

FIGS. 3 to 5 show a partial view of the device from FIG. 1 in the region of the build platform and the vat bottom in successive working steps,

FIG. 6 shows a plan view from above of a second embodiment of the invention,

FIG. 7 shows a lateral plan view, partly in section, of the device of the second embodiment from FIG. 6, and

FIG. 8 shows a plan view from above of a third embodiment of the device.

DETAILED DESCRIPTION OF THE INVENTION

The device according to the invention is characterized in that the vat is horizontally movable with respect to the exposure unit and the build platform and in that a feed device is provided and, under the control of the control device, discharges light-polymerizable material into the vat, the exposure unit and the build platform being arranged horizontally at a distance from the feed device, and in that the control unit is arranged for moving the vat between successive exposure steps in a prescribed way into a horizontal plane by a drive in order to bring light-polymerizable material that has been discharged in this way by the feed device onto the vat bottom into the region between the exposure unit and the build platform.

In a preferred embodiment, arranged between the feed device and the exposure unit/build platform in the direction of movement of the vat is an application device, in particular a doctor blade or a roller, the height of which above the vat bottom can be set, in order to act on the layer of light-polymerizable material discharged by the feed device and bring it to a uniform thickness before it reaches the intermediate space between the exposure unit and the build platform.

In a preferred embodiment, the exposure unit is arranged below the vat bottom for exposure of the at least partially transparent or translucent vat bottom from below, and the build platform is held in a lifting mechanism above the vat bottom in a manner in which it can be adjusted in height by the control unit.

Preferably, the control unit is arranged for setting the thickness of the layer, to be specific the distance between the build platform or the layer last formed and the vat bottom, by means of the lifting mechanism.

In the lifting mechanism, and connected to the control unit, there is preferably a force transducer, which is capable of measuring the force exerted by the lifting mechanism on the build platform and sending the measurement result to the control unit, the control unit being arranged for moving the build platform with a prescribed force profile. In particular in the case of ceramic-filled light-polymerizable materials, on account of the high viscosity, great forces may occur when the build platform is moved down into or moved up out of the viscous material, caused by the viscous material being displaced from or sucked in between the build platform and the vat bottom. In order to limit the forces occurring and nevertheless allow the highest possible lowering and raising rates, which speeds up the production process as a whole, the control unit may use the lifting mechanism optimally in a force-controlled manner by force measurement.

To perform the horizontal movement between successive exposure steps, the vat may be mounted with its bottom rotatable about a central axis and be turned by a prescribed angle by a drive between successive exposure steps. The exposure unit and the build platform lying above it lie offset radially outward with respect to the central axis, so that in successive exposure steps and rotational movement steps taking place in between the vat bottom is ultimately passed over in the form of a circular ring. The application device, for example a doctor blade or roller or combinations thereof, then lies between the feed device on the one hand and the exposure unit and the build platform on the other hand in the direction of movement, so that the exposure process takes place after the application device has acted on the layer of material. Multiple doctor blades or rollers or combinations thereof may be provided in order to have a smoothing and rolling effect on the layer. The application device may also be formed in particular by an edge of a discharge channel of the feed device, which lies at a settable height above the vat bottom.

As an alternative to the rotational movement, the vat may be mounted such that it can be moved in a line and a drive that is capable of moving the vat by a prescribed distance between successive exposure steps under the control of the control unit may be provided.

By suitable choice of the size of the movement steps of the vat, strategies which allow the vat bottom to be exposed at new places each and every time can be carried out, so that adhesive attachment of the light-polymerizable material to the vat bottom caused by repeated exposure of the same place on the vat bottom can be reduced. In a rotational movement of the vat, for example, the ratio of full circle)(360° to rotational angle increment is preferably not an integral number, in particular also not a rational number. Alternatively, the rotational angle increments may also be varied in a prescribed or random manner, so that the polymerization always takes place in different regions of the vat.

Light-emitting diodes are preferably used in the device as the light source of the exposure unit and/or of the further exposure unit. Conventionally, mercury vapor lamps have been used in the case of stereolithography processes with mask projection, which however entails disadvantages since the luminous density of such mercury vapor lamps can vary considerably over time and space, which often makes repeated calibrations necessary. It is therefore preferred to use light-emitting diodes, which show significantly lower variations in intensity over space and time. Nevertheless, in a preferred embodiment, the device is arranged for carrying out a correction or compensation of variations in intensity automatically at prescribed intervals. For this purpose, it may be provided that the exposure unit has a reference sensor, which is formed as a photosensor scanning the entire exposure area or as a CCD camera recording the entire exposure area. The control unit is arranged for operating in a calibration step by exposing the exposure unit above the exposure area with a control signal for the exposure unit that is prescribed over the entire exposure area and using the intensity pattern recorded by the reference sensor in the case of the prescribed control signal for calculating a location-dependent compensation, the application of which produces a uniform intensity in the entire exposure area. In other words, the compensation mask delivers location-dependently in the exposure area a relationship between the signal amplitude controlling the exposure unit and the actual intensity respectively resulting from this. This allows time-dependent or permanently occurring variations of the locational intensity distribution in the exposure area to be compensated, by the exposure unit being controlled location-dependently by the control unit with a mask inverse to the intensity pattern actually recorded in the case of the prescribed control signal in the last calibration step, so that a uniform actual intensity can be achieved in the exposure area.

Light-emitting diodes which emit light with different optical wavelengths are preferably used. This makes it possible to process different materials with different photoinitiators in the same device.

The exposure unit is preferably designed for the emission of light with an average intensity of 1 mW/cm² to 2000 mW/cm², in particular 5 mW/cm² to 50 mW/cm².

The exposure unit preferably has a spatial light modulator, in particular a micromirror array controlled by the control unit.

In a preferred embodiment, behind the region of the exposure unit and the build platform there lies a wiper which can be positioned at a prescribable height above the vat bottom and is designed for renewed distribution of the material after the polymerization process. After an exposure step and after the build platform has been raised, a zone without light-polymerizable material, corresponding to the form of the layer last formed, is left behind in the layer of material on the vat bottom. This zone is filled again at the latest when it is passed by the wiper, by renewed distribution of the material on the vat bottom.

The device is preferably designed for the purpose of performing a relative tipping movement between the build platform and the vat bottom when the raising of the build platform is initiated after an exposure step, under the control of the control unit, whereby a more gentle separation of the layer of polymerized material from the vat bottom is achieved, and consequently less stress on the object.

In a preferred embodiment there are a plurality of vats, each of which is assigned a feed device for one of a plurality of light-polymerizable materials, and a drive, which, under the control of the control unit, is capable of moving one of the vats in each case in a selected prescribed sequence between the exposure unit and the build platform, this movement being a linear movement in the case where multiple vats are arranged in series or a rotating movement in the case where multiple vats are arranged along a curved path, whereby layers of different materials can be built up in accordance with the selected prescribed sequence. The different materials may differ in their colour and other material properties, in particular optical and mechanical properties, in order in this way to achieve a desired layering of materials in the object. Preferably, when changing from one vat to another, the entrainment of material from one vat into the other vat is suppressed, in order to avoid contamination of the materials in the vats. For this purpose, it may be provided in particular that, after raising from one vat, the build platform with the layers already formed on it is taken through a cleaning device, in particular through a bath with solvent, in order to remove adhering non-polymerized material.

A vat in the sense of the embodiment described in the previous paragraph means a divided, upwardly open receiving space for light-polymerizable material. In particular, therefore, a single vat body may also be divided by separating walls into a number of vat segments that are separate from one another, in order in this way to form a plurality of vats in the sense of this invention.

The feed device preferably has a receptacle for inserting a cartridge with light-polymerizable material, in order to be able in a simple way to use the light-polymerizable material that is desired for the respective building process.

The underside of the build platform is preferably provided with a structuring, for example comprising nubs, channels or grooves, which is provided in or on the lower surface itself and/or in or on a coating or film applied thereto. The at least partially transparent or translucent vat bottom is preferably formed by a film or a sheet containing a polymerization inhibitor. The build platform may, in particular, consist of a high-temperature-resistant material, preferably of zirconium oxide, aluminum oxide, sapphire glass or quartz glass.

A method according to the invention of the aforementioned type is characterized in that the at least one vat can be moved horizontally into a feed position, the feed device discharges light-polymerizable material at least onto an exposure area of the vat bottom before the vat is moved into an exposure position, in which the exposure area is located below the build platform and above the exposure unit, and the exposure takes place.

In a preferred embodiment, the distribution of the fed light-polymerizable material takes place in a prescribed layer thickness on the vat bottom, in particular on the exposure area, while the moving of the vat from the feed position into the exposure position is performed with the aid of an application device, for example a doctor blade or a roller, the height of which above the vat bottom can be set, arranged between the feed device and the exposure unit as well as the build platform.

Preferably, the build platform, possibly with the layers already formed on it is preferably lowered again by the lifting mechanism under the control of the control unit into the newly fed light-polymerizable material, so that light-polymerizable material is displaced from the remaining intermediate space with respect to the vat bottom, and the distance between the lower surface of the layer last formed and the vat bottom is set in a prescribed manner by the control unit. In this way, the thickness of the layer to be formed, which corresponds to the distance between the lower surface of the layer last formed and the vat bottom, can be precisely set by mechanically precise setting of the build platform above the vat bottom.

The first layer of light-polymerizable material is preferably polymerized onto a, possibly removable, film or coating arranged on the underside of the build platform.

The displacement of the build platform preferably takes place by raising and/or lowering under force control in accordance with a prescribed force profile, i.e. the force exerted by the lifting mechanism on the build platform is limited with respect to prescribed criteria. As a result, the forces occurring, which may be considerable, particularly in the case of materials of relatively high viscosity, and could detrimentally affect the buildup of the object, can be limited while nevertheless allowing the highest possible lowering and raising rates of the build platform into and out of the light-polymerizable material, which optimizes the speed of the production process as a whole, since it is possible to work at all times with the highest speed at which detrimental effects are still avoided.

To allow the build up of objects using different materials, a plurality of different materials can be used for building up layers in a selectable sequence in successive layer building steps, by a plurality of vats, each assigned a feed device with one of the plurality of materials, being moved in a selected sequence between the exposure unit and the build platform, this movement being a linear movement in the case where multiple vats are arranged in series or a rotating movement in the case where multiple vats are arranged along a curved path. The different materials may differ in their color and other material properties, in particular optical and mechanical properties, in order in this way to achieve a desired layering of materials in the object.

In a preferred embodiment, a particle-filled, for example ceramic-filled, light-polymerizable material is used for the production of the object and the organic constituents are burned out from the finished object before the object is sintered. The particle fraction of the light-polymerizable material may preferably consist of an oxide ceramic or a glass ceramic.

The light-polymerizable material on the underside of the build platform is preferably polymerized by exposure from below, after which the build platform is raised in relation to a vat for the light-polymerizable material after each exposure step and light-polymerizable material is newly fed under the layer last formed. In this case, the first layer of light-polymerizable material may be polymerized onto a removable film or coating arranged on the underside of the build platform.

The object to be produced by the method according to the invention may be, for example, a green blank for a dental restoration, in which case the light-polymerizable material may be, for example, a ceramic-filled photopolymer. The build platform preferably has a sheet of a high-temperature-resistant material, preferably of zirconium oxide, aluminum oxide, sapphire glass or quartz glass. A transparent polymer film may be adhesively attached on such a ceramic base in order to form the build platform, it being possible for the polymer film to be provided with structurings such as nubs, channels or the like on the side that comes into contact with the photopolymer, in order to achieve better adhesive attachment of the ceramic-filled photopolymer. After the successive buildup of the green blank, the build platform with the green blank adhesively attached thereto can be removed and introduced directly into the sintering furnace. During the debinding of the component, not only the organic resin component but also the polymer film of the build platform decomposes, and after the sintering the sintered ceramic object consequently lies loosely on the sheet of the build platform and can be removed.

In the case of the method according to the invention, a plastic may preferably be used for producing the object, the object being embedded in an embedding compound after it has been produced and burned out after the embedding compound has solidified, and a different material, in particular a dental ceramic material or metal or an alloy, being forced into the cavities created in the embedding compound.

In the case of a preferred method, a dental composite may be used for the production of the object and, after it has been produced, the object may be heat-treated and subsequently polished or coated and subsequently heat-treated.

In the case of a method according to the invention, the ceramic fraction of the ceramic-filled photopolymer preferably consists of an oxide ceramic or a glass ceramic, in particular zirconium oxide, aluminum oxide, lithium disilicate, leucite glass ceramic, apatite glass ceramic or mixtures thereof.

In the case of a method according to the invention, after carrying out an exposure step with the vat stationary, the build platform is preferably raised in order to lift off the layer formed from the vat bottom. For this purpose, a slight relative tipping movement between the build platform and the vat bottom is preferably carried out, since, after the polymerization, adhesive attachment of the layer formed to the vat bottom could lead to excessive mechanical stress on the layer just formed or the entire component if it were pulled vertically upward. After the build platform has been raised, a zone without light-polymerizable material, corresponding to the form of the layer last formed, is left behind in the layer of material on the vat bottom. This zone is filled again at the latest when it is passed by the doctor blade or the roller or by an optional additional wiper, by renewed distribution of the material on the vat bottom.

The following exemplary embodiment relates to the production of a green blank for a dental restoration.

Firstly, the main components of the device are described with reference to FIGS. 1 and 2.

In the embodiment represented in FIGS. 1 and 2, the device has a housing 2, which serves for accommodating and fitting the other components of the device. The upper side of the housing 2 is covered by a vat 4, which has, at least in the regions intended for exposures, a transparent or translucent and planar vat bottom.

Provided in the housing 2, under the vat bottom 4, is an exposure unit 10, which can, under the control of a control unit 11, expose a predetermined exposure area on the underside of the vat bottom 6 selectively with a pattern in the desired geometry.

The exposure unit 10 preferably has a light source 15 with multiple light-emitting diodes 23, a luminous power of approximately 15 to 20 mW/cm² preferably being achieved in the exposure area. The wavelength of the light radiated from the exposure unit preferably lies in the range from 400 to 500 nm. The light of the light source 15 is modulated location-selectively in its intensity by way of a light modulator 17 and imaged in the resultant intensity pattern with the desired geometry on the exposure area on the underside of the vat bottom 6. Various types of so-called DLP chips (digital light processing chips) may serve as light modulators, such as for example micromirror arrays, LCD arrays and the like. Alternatively, a laser may be used as the light source, the light beam of which successively scans the exposure area by way of a movable minor, which may be controlled by the control unit.

Provided over the exposure unit 10 on the other side of the vat bottom 6 is a build platform 12, which is held by a lifting mechanism 14 with a carrier arm 18, so that it can be held over the vat bottom 6 above the exposure unit 10 in a height-adjustable manner. The build platform 12 is likewise transparent or translucent.

Arranged above the build platform 12, which is formed such that it is transparent or translucent, is a further exposure unit 16, which is likewise controlled by the control unit 11 in order, at least during the forming of the first layer under the build platform 12, also to irradiate light from above through the build platform 12, in order thereby to achieve dependable and reliably reproducible polymerization and adhesive attachment of the first polymerized layer on the build platform.

Also provided above the surface of the vat 4 is a feed device 8 with a reservoir in the form of an exchangeable cartridge 9 filled with light-polymerizable material. Under the control of the control unit 11, ceramic-filled light-polymerizable material can be successively discharged from the feed device 8 onto the vat bottom 6. The feed device is held by a height-adjustable carrier 34.

The vat 4 is mounted rotatably about a vertical axis 22 on the housing 2 by a bearing 7. A drive 24, which, under the control of the control unit 11, sets the vat 4 in a desired rotational position, is provided.

A wiper 30, which can undertake various functions, as explained further below, may be arranged between the exposure unit 12 and the feed device 8 in the direction of rotation, at a settable height above the vat bottom 6.

As can be seen from FIG. 2, lying between the feed device 8 and the exposure unit 12, above the vat bottom 6, is an application device 26, here in the form of a doctor blade 26, which can be positioned at a settable height above the vat bottom 6, in order in this way to smooth material that has been discharged from the feed device 8 onto the vat bottom 6 by moving past the application device before it reaches the exposure unit 12, in order thereby to ensure a uniform distribution and prescribed layer thickness. Alternatively or in addition to the doctor blade, one or more rollers or further doctor blades may belong to the application device, in order to act in a smoothing manner on the layer of material.

The pivot arm 18 carrying the build platform 12 is connected by way of a pivot joint 20 to the vertically displaceable part of the lifting mechanism 14. Also provided in the lifting mechanism 14 is a force transducer 29, which measures the force exerted by the lifting mechanism 14 on the build platform 12 during the lowering and raising thereof and sends the measurement result to the control unit 12. As described further below, said control unit is designed for the purpose of controlling the lifting mechanism 14 on the basis of a prescribed force profile, for example to limit the force exerted on the build platform 12 to a maximum value.

The way in which the device represented in FIGS. 1 and 2 functions can be summarized as follows. Under the control of the control unit, a prescribed amount of ceramic-filled light-polymerizable material 5 is discharged from the feed device 8 onto the vat bottom 6. By controlling the drive 24, the control unit 11 instigates a turning of the vat bottom 6 about the axis of rotation 22, so that the discharged material passes the application device 26, here a doctor blade, which smooths the light-polymerizable material to a prescribed layer thickness 32, which is determined by the height setting of the application device 26. Furthermore, by turning of the vat 4, the material is brought into the region between the build platform 12 and the exposure unit 10.

After stopping the turning movement of the vat 4, here there then follows the lowering of the build platform 12 into the layer of light-polymerizable material 5 formed on the vat bottom 6, which is explained below on the basis of FIGS. 3 to 5. In the state shown in FIG. 3, a layer of light-polymerizable material 5 with a prescribed thickness 32 is formed on the vat bottom, the build platform 12 still being located above the layer 5 in this state. Attached to the underside of the build platform 12 is a film 13, which will be discussed further below. From the state represented in FIG. 3, a lowering of the build platform 12 then takes place by the lifting mechanism 14 under the control of the control unit 11, so that the build platform 12 with the film 13 on the underside is immersed into the layer of light-polymerizable material 5 and, as it is lowered further, displaces said layer partially out of the intermediate space between the film 13 and the upper surface of the vat bottom 6. Under the control of the control unit 11, the build platform 12 is lowered by the lifting mechanism 14 to the vat bottom in such a way that a layer with a precisely prescribed layer thickness 21 is defined between the build platform and the vat bottom. As a result, the layer thickness 21 of the material to be polymerized can be precisely controlled.

During the immersion of the build platform 12 into the light-polymerizable material 5 and the further lowering into the position shown in FIG. 4, great forces can occur, particularly when material of relatively high viscosity is displaced, if the lowering of the build platform were to take place at the prescribed rate. In order to prevent the layers of material that build up during the lowering of the build platform 12 into the light-polymerizable material 5 from being exposed to great forces, in the lifting mechanism there is the aforementioned force transducer 29, which measures the force exerted on the build platform 12 and sends the measurement signal to the control unit 11. This control unit is arranged for controlling the lifting mechanism in such a way that the force recorded by the force transducer 29 follows prescribed criteria, in particular that the force exerted does not exceed a prescribed maximum force. As a result, on the one hand the lowering of the build platform 12 into the light-polymerizable material 5 and the raising of the build platform out of said material can be carried out in a controlled manner such that the forces exerted on the build platform, and consequently also on the layers already formed, are limited and, as a result, detrimental effects are avoided during the buildup of the object, and on the other hand the lowering and raising of the build platform 12 can be carried out at the maximum possible rate at which detrimental effects on the object to be built up are still just avoided, in order in this way to achieve an optimal process rate.

After the lowering of the build platform into the light-polymerizable material 5, into the position shown in FIG. 4, there then follows the first exposure step for the polymerization of the first layer 28 on the build platform 12, the further exposure unit 16 thereby also being actuated (at the same time or with a time delay), in order to ensure reliable adhesive attachment of the first polymerization layer 28 to the build platform. During the exposure process, the vat 4 is kept stationary, i.e. the drive 24 remains switched off. After the exposure of one layer, the build platform 12 is raised by the lifting mechanism 14. In this case, however, before the raising of the build platform 12, a relative tipping movement between the build platform 12 and the vat bottom 6 is preferably carried out first. This slight tipping movement is intended to serve the purpose of detaching the last-polymerized layer of the object 27 from the vat bottom 6 with less mechanical stress. After this tipping movement and detachment of the layer last formed, the build platform is raised by a prescribed amount, as shown in FIG. 5, so that the layer last formed lies above the light-polymerizable material 5 on the object 27.

Subsequently, material is again discharged from the feed device 8 and the vat 4 is turned by a prescribed rotational angle by the drive 24, the material that moves past the doctor blade again being brought to a uniform layer thickness. This series of steps, with the forming of successive layers of a predetermined form of contour, is continued until the succession of layers with respectively predetermined geometry provides the desired form of the ceramic green blank.

The wiper 30, provided behind the exposure unit and above the vat bottom 6, may have various functions. For example, when it has been lowered fully onto the vat bottom 6, it may serve the purpose of collecting the material from the vat bottom and carrying it away or returning it into the feed device 8, which should take place at the end of a building process. During a building process, when it is raised slightly with respect to the vat bottom 6, the wiper 30 serves the purpose of distributing the material again, in particular pushing the material back into the “holes” that have been created in the layer of material by the exposure process after raising of the build platform 12.

After the completion of a building process, the build platform 12 with the exposure unit 16 fitted above it, can be pivoted upward as a whole by pivoting the pivot arm 18 about the joint 20, as indicated by dashed lines in FIG. 1. After that, there is better access to the vat 4, for example to allow the latter to be cleaned or exchanged.

After the described buildup of the green blank from polymerized ceramic-filled material, said blank must be removed from the device and fed to a firing furnace, in which a decomposition of the polymerized binder (debinding) is brought about by the thermal treatment and a sintering of the ceramic material is carried out. To simplify the handling of the built-up body, the build platform is designed such that it can be easily detached from the carrier arm 18. Then the build platform, with the built-up ceramic-filled object 27 adhesively attached to it, can be removed from its carrier 18 and placed in a firing furnace. In order to allow this preferred simple removal of the built-up dental restoration element of ceramic-filled polymer, the build platform must however be produced from a high-temperature-resistant material, for which zirconium oxide, aluminum oxide, sapphire glass or quartz glass may serve for example. Possible as an alternative to this is a self-adhesive, transparent film, which may be structured with nubs, channels, scores etc. on the side facing the photopolymer, for better adhesive attachment, and can be removed after the building process by simple detachment from the build platform or together with the build platform and passed together with the film into the firing furnace for debinding/sintering.

As compared with the device from FIGS. 1 and 2 with a rotatable vat, FIGS. 6 and 7 show an alternative embodiment, in which the vat 54 is designed such that it is movable linearly back and forth. In this embodiment, a vat 54 is mounted linearly movably on the housing 52 in a bearing 57. Above the vat 54, the feed device 58 is arranged in a height-adjustable manner. Offset from the feed device 54 with respect to the linear direction of movement, the build platform 62 is held above the vat 54 on a pivot arm 68, which belongs to a lifting mechanism 64. The pivot arm 68 is in turn provided with a pivot joint 70, which allows the pivot arm 68, after being raised in the vertical direction, to be turned by 180°, after which the build platform 62 with the object built up on it faces upward, and in this position can be easily handled.

Located below the build platform 62 and the vat bottom 56 is the exposure unit 60, in which a light source 65 with light-emitting diodes 73 is arranged. The light of the light source 65 is projected by way of a light modulator 67 and through the transparent vat bottom 56 onto the build platform 62. Also present in the exposure unit 60 is a reference sensor 51, which is used in a calibration step for the purpose of recording the actual intensity distribution in the exposure area when the light modulator is controlled in such a way as to obviate any locational dependence or modulation over the exposure area. From the deviation of the intensity distribution actually recorded, a control profile (compensation mask) for the light modulator can then be calculated by inversion and provide an intensity that is actually uniform over the exposure area. A corresponding reference sensor 1 is also present in the case of the embodiment from FIGS. 1 and 2.

Arranged one behind the other in the direction of movement of the vat 54 (indicated by the double-headed arrow in FIGS. 6 and 7) are an application device 76, which is held in a height-adjustable manner above the vat bottom 56 and here is in the form of a doctor blade, the lower edge of which lies at a suitable distance from the surface of the vat bottom, and a wiper 80.

The way in which the device shown in FIGS. 6 and 7 functions corresponds to the method steps described above with reference to FIGS. 3 to 5, apart from the difference of the linear movement back and forth of the vat 54 instead of the rotating movement of the vat 4. Firstly, instigated by the control unit 61, which actuates the drive 75, the vat 54 is displaced from the position shown in FIG. 7 to the left into the position shown by dashed lines (feed position). In this case, light-polymerizable material is discharged by the feed device 58 onto the vat bottom 56, the amount and variation over time of the discharge likewise being prescribed by the control unit 61. After that, by reversing the drive 75, the control unit 61 causes the vat 54 to be displaced back again. As this happens, the light-polymerizable material 55 discharged onto the vat bottom 56 first passes the wiper 80 and then the application device 76, which ensure a uniform distribution and uniform layer thickness of the light-polymerizable material 55, before it reaches the intermediate space between the build platform 62 and the exposure unit 60. After that, the drive 75 is stopped, whereupon the series of steps as described above in conjunction with FIGS. 3 to 5 is executed, the build platform 62 being immersed into the layer of light-polymerizable material 55 and a layer with prescribed thickness being defined between the build platform and the vat bottom by setting of the distance from the vat bottom. After that, the actuation of the exposure unit 60 takes place to generate an exposure pattern with predetermined geometry, the further exposure unit 66 with its light-emitting diodes 69 also being actuated in this connection, at least during the generation of the first layer directly on the build platform 62, in order to expose the first layer by exposure through the transparent or translucent build platform and thereby achieve complete polymerization and reliable adhesive attachment of the first layer to the build platform 62.

After polymerization of the first layer with the desired geometry, the build platform 62 is raised again by actuating the lifting mechanism 64, so that the polymerized layer formed is raised above the level of the light-polymerizable material 55.

After that, the series of steps described is repeated, i.e. the vat 54 is again displaced to the left, light-polymerizable material is discharged from the feed device 58 and this material is distributed uniformly by the wiper 80 and the application device 76 when the vat 54 is pushed back to the right, after which, by switching off the drive 75, the lifting mechanism 64 lowers the build platform 62 again, so that the last-formed polymerized layer is immersed into the light-polymerizable material 55 and is brought to a prescribed distance above the vat bottom 56, in order to polymerize the layer of material that is then lying in the intermediate space in the next exposure step. The increment of the movement back and forth can of course be varied again, in order to avoid polymerization always being carried out over the same place on the vat bottom.

The lifting mechanism 64 is in turn provided with a force transducer 79, the measured values of which are used by the control unit 61 in the way described above in connection with the first embodiment for limiting the force that is exerted on the build platform during the lowering and raising of the build platform.

Methods in which multiple different ceramic-filled photopolymerizable materials are used for building up the green blank may preferably also be used. This may take place, for example, by a plurality of vats being provided, each with an assigned reservoir with different materials. These vats may then be moved to the exposure unit and the build platform in the manner of a changeover carrier, in order to process different materials in a prescribed sequence. For this purpose, the multiple vats may, for example, be arranged in series one behind the other on a carrier, which is then linearly movable with respect to the exposure unit and the build platform, in order to provide a desired vat in each case. Alternatively, a plurality of rotatable vats, one of which is represented in FIGS. 1 and 2, may be arranged on a circular ring of a larger plate, which for its part is in turn rotatable, in order in each case, by setting the rotational position of the plate, to bring a desired vat into the position between the exposure unit and the build platform in which the polymerization step of the respective layer is then carried out.

A special embodiment of a device with which various light-polymerizable materials can be used for building up an object is shown in FIG. 8 in a schematic plan view from above. Here there are four vats 104 on a turntable, arranged in the form of a circular ring. The arrangement of the feed device 108, the further exposure unit 116 on a lifting mechanism 114 as well as the wiper 130 and the application device 126 lying in between is largely similar to the arrangement of the device from FIGS. 6 and 7, with the exception of the fact that the components are not arranged along a linear path and the vat is not linearly movable, but instead the components are arranged along a segment of a circular ring and the vat correspondingly has the form of a segment of a circular ring. Between successive exposure steps in the same vat 104, the vat is moved back and forth by an angle of approximately less than 90°, so that in turn a movement back and forth is obtained between the feed device 118 and the build platform located under the further exposure unit 108.

If at a specific point in time one of the materials from one of the three other vats 104 is to be used, the turntable is turned by an angle corresponding to 90°, 180° or 270°, in order to bring one of the following vats to the device under consideration for building up the object.

As indicated at the bottom in FIG. 8, a further device for building up objects, which can operate in parallel with the device shown at the top, may be provided on the turntable in the region of another segment of a circular ring.

Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims. 

The invention claimed is:
 1. A method for processing light-polymerizable material (5; 55; 105) for building up an object (27), using a lithography-based generative fabrication technique, in which a layer (28) of a light-polymerizable material (5; 55; 105), which is located in at least one vat (4; 54; 104) with transparently or translucently formed, horizontal bottom (6; 56; 106), is polymerized on at least one horizontal build platform (12; 62), the horizontal build platform (12; 62) protruding into the at least one vat (4; 54; 104), with predetermined geometry by exposure in an exposure area, the build platform (12; 62) is displaced vertically for forming of a subsequent layer, light-polymerizable material (5; 55; 105) is newly fed under a layer (28) last formed, and by repeating the previous steps, the object (27) is built up in layers in a form, which results from a sequence of layer geometries, wherein the at least one vat (4; 54; 104) can be moved horizontally into a feed position, a feed device (8) discharges light-polymerizable material (5; 55; 105) at least onto the exposure area of at least one vat bottom (6; 56; 106) before the at least one vat (4; 54; 104) is moved into an exposure position, in which the exposure area is located below the build platform (12; 62) and above an exposure unit (10; 60), and exposure takes place, wherein a layer thickness of the material (5; 55; 105) is set by a distance between the build platform (12; 62) and the at least one vat bottom (6; 56; 106) with the aid of a lifting mechanism (14; 64; 114), which, after renewed feeding of light-polymerizable material, lowers the build platform (12; 62) with layers formed on the build platform, again into a newly fed light-polymerizable material (5; 55; 105), so that light-polymerizable material is displaced from a remaining intermediate space with respect to the at least one vat bottom (6; 56; 106), and a remaining distance between a lowered lower surface and the vat bottom is set in a manner under control of a control unit (11; 61).
 2. The method as claimed in claim 1, wherein distribution of the fed material (5; 55; 105) takes place in a prescribed layer thickness (33) on the at least one vat bottom (6; 56; 106), on the exposure area, while moving of the at least one vat (4; 54; 104) from a feed position into the exposure position is performed with aid of an application device (26; 76; 126) arranged between the feed device (8; 58; 108) and the exposure unit (10; 60) as well as the build platform (12; 62), a height of which above the at least one vat bottom (6; 56; 106) can be set.
 3. The method as claimed in claim 1, wherein raising and/or lowering of the build platform (12; 62) takes place under force control in accordance with a prescribed force profile.
 4. The method as claimed in claim 1, wherein the at least one vat (4) is rotatably mounted and is turned about a prescribed angle about the vertical axis (22) between successive layer building steps.
 5. The method as claimed in claim 1, in which the at least one vat (54) is mounted such that it can be moved laterally and is moved horizontally over a prescribed distance between successive layer building steps.
 6. The method as claimed in claim 1, in which a plurality of different materials (105) are used for building up layers in a selectable sequence in successive layer building steps, by a plurality of vats (104), each assigned a feed device (108) with one of the plurality of materials (105), being moved in a selected sequence between the exposure unit and the build platform, this movement being a rotating movement where multiple vats (104) are arranged along a curved path or a linear movement where the vats are arranged along a straight line.
 7. The method as claimed in claim 1, wherein the lithography-based generative fabrication technique comprises rapid prototyping.
 8. The method as claimed in claim 2, wherein the application device comprises a doctor blade or a roller. 